Undesirable acidic species such as mercaptans may be removed from liquid hydrocarbons with conventional aqueous treatment methods. In one conventional method, the hydrocarbon contacts an aqueous treatment solution containing an alkali metal hydroxide. The hydrocarbon contacts the treatment solution, and mercaptans are extracted from the hydrocarbon to the treatment solution where they form mercaptide species. The hydrocarbon and the treatment solution are then separated, and a treated hydrocarbon is conducted away from the process. Intimate contacting between the hydrocarbon and aqueous phase leads to more efficient transfer of the mercaptans from the hydrocarbon to the aqueous phase, particularly for mercaptans having a molecular weight higher than about C4. Such intimate contacting often results in the formation of small discontinuous regions (also referred to as “dispersion”) of treatment solution in the hydrocarbon. While the small aqueous regions provide sufficient surface area for efficient mercaptan transfer, they adversely affect the subsequent hydrocarbon separation step and may be undesirably entrained in the treated hydrocarbon.
Efficient contacting may be provided with reduced aqueous phase entrainment by employing contacting methods that employ little or no agitation. One such contacting method employs a mass transfer apparatus comprising substantially continuous elongate fibers mounted in a shroud. The fibers are selected to meet two criteria. The fibers are preferentially wetted by the treatment solution, and consequently present a large surface area to the hydrocarbon without substantial dispersion or the aqueous phase in the hydrocarbon. Even so, the formation of discontinuous regions of aqueous treatment solution is not eliminated, particularly in continuous process.
In another conventional method, the aqueous treatment solution is prepared by forming two aqueous phases. The first aqueous phase contains alkylphenols, such as cresols (in the form of the alkali metal salt), and alkali metal hydroxide, and the second aqueous phase contains alkali metal hydroxide. Upon contacting the hydrocarbon to be treated, mercaptans contained in hydrocarbon are removed from the hydrocarbon to the first phase, which has a lower mass density than the second aqueous phase. Undesirable aqueous phase entrainment is also present in this method, and is made worse when employing higher viscosity treatment solutions containing higher alkali metal hydroxide concentration.
There remains a need, therefore, for continuous hydrocarbon treatment processes that curtail aqueous treatment solution entrainment in the treated hydrocarbon, and are effective for removing acidic species such as mercaptan, especially high molecular weight and branched mercaptans.